The present disclosure generally relates to wind turbines, and more particularly, to a monitor for measuring stress in a wind turbine blade during operation of the wind turbine as well as to processes for monitoring stress in a wind turbine blade.
Wind turbines generally convert the mechanical energy captured by the rotating wind turbine blades into electrical energy using a generator. A wind turbine, like some other structures such as aircraft propellers, fans, and the like, includes blades configured for rotating about an axis. Typically, two or more blades are provided each coupled to a rotatable hub. Most turbines have either two or three blades. Wind blowing over the blades causes the blades to lift and rotate. During this conversion, the wind turbine blades can be exposed to relatively large and variable loads during their operation. Because wind is a natural force and cannot be controlled, the wind turbine must withstand exposure to varying wind conditions from no wind at all to winds in excess of 100 miles per hour.
Cyclic stresses can fatigue the blade, axle and bearing materials, and have been a cause of turbine failure for many years. Components that are subject to repeated bending, such as rotor blades, may eventually develop cracks that ultimately may make the component break. Current monitoring processes for measuring the stress applied to the wind turbine blades employ the use of strain gages. The strain gauges are typically flat electrical resistors that are glued onto the surface of the rotor blades being tested and are used to measure very accurately the bending and stretching behavior of the rotor blades. The measurement that results from the strain gauges can be continuously monitored on computers. Nonlinear variations in the pattern of bending may reveal damage to the rotor blade structure.
Other methods for monitoring blade fatigue include infrared analysis (also referred to as thermography). In these methods, infrared cameras are typically used to reveal local build-up of heat in the blade. The heat build-up may either indicate an area with structural dampening, i.e. an area where the blade designer has deliberately laid out fibers which convert the bending energy into heat in order to stabilize the blade, or it may indicate an area of delamination or an area which is moving toward the breaking point for the fibers. In this manner, catastrophic failure can be prevented.
Rotor blades are also tested for strength (and thus their ability to withstand extreme loads) by being bent once with a very large force. This test is made after the blades has been subject to fatigue testing, in order to verify the strength for a blade which has been in operation for a substantial amount of time.
Although the above noted monitoring processes are suitable for their intended use, they are generally complex in nature or cannot be implemented during actual operation of the wind turbine. Accordingly, it is desirable to have a relatively simple method for monitoring stress in the wind turbine blade so as to prevent the occurrence of catastrophic failures.